The present invention relates generally to a superconductive magnet used to generate a uniform magnetic field, and more particularly to such a magnet having a pole piece.
Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics and procedures. Known superconductive magnets include liquid-helium-cooled and cryocooler-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a first thermal shield surrounded by a vacuum vessel. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum vessel, having its first cold stage in thermal contact with the thermal shield, and having its second cold stage in thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium dewar surrounding the superconductive main coil and a second thermal shield which surrounds the first thermal shield which surrounds the liquid-helium dewar.
Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet""s bore where the object to be imaged is placed. A single, tubular-shaped shielding assembly may also be used to prevent the high magnetic field created by and surrounding the main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Such shielding assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive shielding coils carrying electric currents of generally equal amperage, but in an opposite direction, to the electric current carried in the main coils and positioned radially outward of the main coils.
Open magnets, including xe2x80x9cCxe2x80x9d shape magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume and allowing for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. Known open magnet designs having shielding include those wherein each coil assembly has an open bore and contains a resistive or superconductive shielding coil positioned longitudinally and radially outward from the resistive or superconductive main coil(s). In the case of a superconductive magnet, a large amount of expensive superconductor is needed in the main coil to overcome the magnetic field subtracting effects of the shielding coil. Calculations show that for a 0.75 Tesla magnet, generally 2,300 pounds of superconductor are needed yielding an expensive magnet weighing generally 12,000 pounds. The modest weight makes this a viable magnet design.
It is also known in open magnet designs to place an iron pole piece in the bore of a resistive or superconductive coil assembly which lacks a shielding coil. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving the homogeneity of the magnetic field. An iron return path is used to connect the two iron pole pieces. It is noted that the iron pole piece also acts to shield the magnet. However, a large amount of iron is needed in the iron pole piece to achieve shielding in strong magnets. In the case of a superconductive magnet, calculations show that for a 0.75 Tesla magnet, only generally 200 pounds of superconductor are needed yielding a magnet weighing over 70,000 pounds which is too heavy to be used in medical facilities such as hospitals. The weight does not make this a viable magnet design.
What is needed is a superconductive magnet design which is physically more compact and which provides greater magnetic field homogeneity within the magnet""s imaging volume than known designs.
In a first expression of an embodiment of the invention, a superconductive magnet includes a longitudinally-extending axis and a first assembly having a superconductive main coil and a magnetizable pole piece. The main coil is generally coaxially aligned with the axis, carries a first main electric current in a first direction, and is positioned a first radial distance from the axis. The pole piece is generally coaxially aligned with the axis and is spaced apart from the main coil of said first assembly. Most of the pole piece of said first assembly is disposed radially inward of the main coil of said first assembly, and the pole piece of said first assembly extends from the axis radially outward a distance equal to at least 75 percent of the first radial distance. During operation of the magnet, the pole piece of the first assembly has a temperature equal generally to that of the main coil of the first assembly.
In a second expression of an embodiment of the invention, a superconductive magnet includes a longitudinally-extending axis and includes a first assembly having a superconductive main coil, a magnetizable pole piece, and a cryogenic-fluid dewar. The main coil is generally coaxially aligned with the axis and caries a first main electric current in a first direction. The pole piece is generally coaxially aligned with the axis, is spaced apart from the main coil of the first assembly, and has a surface portion. Most of the pole piece of the first assembly is located radially inward of the main coil of the first assembly. The dewar encloses the main coil of the first assembly and has an interior surface defined in part by the surface portion of the pole piece of the first assembly.
In a third expression of an embodiment of the invention, a superconductive open magnet includes a longitudinally-extending axis and longitudinally spaced-apart first and second assemblies each having a superconductive main coil, a superconductive shielding coil, a magnetizable and generally cylindrical-shaped pole piece, and a cryogenic-fluid dewar. Each main coil is generally coaxially aligned with the axis and carries a first main electric current in the same first direction. Each pole piece is generally coaxially aligned with and intersects the axis, is spaced apart from its associated main coil, and has a surface portion. Most of each pole piece is located radially inward of its associated main coil. Each dewar encloses its associated main and shielding coils and has an interior surface defined in part by the surface portion of its associated pole piece.
In one construction, the open magnet also includes spaced-apart and generally-nonmagnetizable first and second support posts each having a first end structurally attached to the pole piece of the first assembly, each having a second end structurally attached to the pole piece of the second assembly, and each having a surface portion. In this construction, the open magnet further includes first and second dewar conduits each in fluid communication with the dewar of the first assembly and the dewar of the second assembly. Here, the first dewar conduit has an interior surface defined in part by the surface portion of the first support post, and the second dewar conduit has an interior surface defined in part by the surface portion of the second support post.
A first method of the invention includes steps a) and b) and provides a homogeneous magnetic resonance imaging volume for a superconductive magnet having a magnetizable pole piece and a superconductive main coil, wherein the main coil has a critical temperature. Step a) includes cooling the main coil to a temperature equal to or less than the critical temperature. Step b) includes cooling the pole piece to a temperature equal to generally the temperature of the main coil.
A second method of the invention includes steps a) through d) and provides both physical compactness and a homogeneous imaging volume for a superconductive magnet having a magnetizable pole piece and a superconductive main coil. Step a) includes obtaining a generally nonmagnetizable coil support. Step b) includes attaching the coil support to the pole piece. Step c) includes supporting the main coil with the coil support. Step d) includes constructing and positioning a cryogenic-fluid dewar to surround the main coil and to have an interior surface defined in part by a surface portion of the pole piece.
A third method of the invention includes steps a) through j) and provides both physical compactness and a homogeneous imaging volume for a superconductive open magnet having a longitudinally-extending axis and having longitudinally spaced-apart and generally coaxially-aligned first and second assemblies each including a magnetizable and generally cylindrical-shaped pole piece intersecting the axis, a superconductive main coil, and a superconductive shielding coil. Step a) includes obtaining generally nonmagnetizable first coil supports. Step b) includes attaching the first coil supports to the pole piece of the first assembly. Step c) includes supporting the main and shielding coils of the first assembly with the first coil supports. Step d) includes constructing and positioning a cryogenic-fluid dewar to surround the main and shielding coils of the first assembly and to have an interior surface defined in part by a surface portion of the pole piece of the first assembly. Step e) includes obtaining generally nonmagnetizable second coil supports. Step f) includes attaching the second coil supports to the pole piece of the second assembly. Step g) includes supporting the main and shielding coils of the second assembly with the second coil supports. Step h) includes constructing and positioning a cryogenic-fluid dewar to surround the main and shielding coils of the second assembly and to have an interior surface defined in part by a surface portion of the pole piece of the second assembly. Step i) includes attaching a first end of a generally-nonmagnetizable support port to the pole piece of the first assembly and attaches a second end of the support post to the pole piece of the second assembly. Step j) includes constructing and positioning a dewar conduit in fluid communication with the dewar of the first assembly and the dewar of the second assembly, wherein the dewar conduit has an interior surface defined in part by a surface portion of the support post.
Several benefits and advantages are derived from the invention. Making the pole piece a cryogenically-cold pole piece provides greater magnetic field homogeneity within the magnet""s imaging volume by eliminating magnetic field inhomogeneities caused by temperature changes of conventional room-temperature pole piece designs caused by changes in room temperature. Making the pole piece a part of the dewar provides physical compactness by eliminating the extra space otherwise required for the cryogenically-cold pole piece of the invention to be completely surrounded by a cryogenic-fluid dewar.